Method of monitoring pollutant emissions of a combustion engine

ABSTRACT

The invention relates to a method for monitoring the pollutant emissions of a combustion engine comprising at least one piston, the translatable movement of which defines a combustion chamber, said engine being combined with an exhaust line that, in the direction of exhaust gas flow, comprises an oxidation catalyst, an NOx reduction catalyst, and a particle filter, the method being characterized by the use of a fuel comprising an additive assisting in soot combustion when re-refined by the particle filter, and according to at least one operating mode of the engine, the fuel is injected in accordance with a calibration minimizing carbon dioxide discharges by the engine.

The present invention claims the priority of French application 0951856 filed on Mar. 24, 2009 the content of which (text, drawings and claims) is incorporated here by reference.

The present invention relates to a method for controlling pollutant emissions of a combustion engine.

The use of fossil fuels such as oil or coal in a combustion system, in particular the fuel in an engine, entails the production in non-negligible quantity of pollutants that can be discharged through the exhaust in the environment, which can cause environmental damage. Among these pollutants, nitrogen oxides (called NOx) pose a specific problem because these gases are suspected of being one of the factors contributing to the development of acid rain and deforestation. Furthermore, NOx gases are linked to human health problems and are one of the key elements in the development of “smog” (pollution clouds) in the cities. The legislation is imposing increasingly rigorous levels for their reduction and/or their elimination from fixed or mobile sources.

Among the pollutants that the legislation tends to regulate more and more strictly are also soot and other particle materials resulting essentially from incomplete combustion of fuel, more particularly when the engine operates in so-called poor mixture, in other words with excess oxygen (air) relative to the stoichiometry of the combustion reaction. Poor mixtures are used in general for diesel engines, which are ignited by compression.

Different depollution means and combustion strategies are implemented for these two main categories of pollutants.

To limit the emission of particles, the technology of particle filters is becoming little by little common for all vehicles equipped with diesel engines. This technology consists in essence in forcing the exhaust gas to pass through the porous channels of a ceramic honeycomb structure. The soot filtered in this manner accumulates and is then eliminated in a regeneration operation of the filter during which the soot is burned. To perform this regeneration, it is however necessary to increase the temperature of the exhaust gas, which is typically obtained by enriching the exhaust gas with fuel (injected directly in the exhaust line or in the combustion chamber of the engine, during the exhaust phase of the combustion cycle) and/or by increasing the charge of the engine. A catalytic agent is used to facilitate the combustion of soot. This agent is either deposited in permanent manner in the filter channels, or introduced as an additive with the fuel. This technology allows for operating at lower combustion temperatures than those required with catalytic filters.

To limit the NOx emissions, the main solution employed in current vehicles consists in reducing the emissions at the source, in other words, operating the engine in such conditions that the rate of produced NOx is lower than the limit rate. These conditions are met in particular by monitoring in very accurate manner the different parameters of the engine, starting from the parameters for fuel injection and re-injection at admission of a portion of the exhaust gas, in order to reduce the oxygen concentration favorable to the development of nitrogen oxides.

Since the tolerated emission levels have a tendency of becoming more severe, another solution consists in using a post-treatment arrangement which introduces a reduction agent in the exhaust line. A post treatment solution which has proven its effectiveness is the use of an ammonia source (NH₃), such as aqueous ureum. The ammonia reacts with the NOx on a catalyst to form inert nitrogen N₂ and water H₂O. This solution is essentially known under its English language acronym SCR “Selective Catalytic Reduction”.

In order to treat both the NOx and the particles, the exhaust line must be equipped with two post-treatment devices, a SCR (S) catalyst and a particle filter (F). The latter requires an oxidation catalyst (C) placed upstream of the filter. Therefore, from upstream to downstream in the direction of the exhaust gas, we can have four architecture types: SCF, CSF, CFS and CSF.

In patent application WO 2007/132102 the advantages are demonstrated of a CSF type architecture, associated with a modulating supervisor during the regeneration phases of the particle filter, the ureum and fuel injections to compensate the thermal loss of the exhaust gas in the catalyst, in this way avoiding that the gas arrives cooled in the particle filter.

In this text, it is simply indicated that the particle filter can be a known type. As indicated previously, there are basically two main types of particle filters, the so-called non-additive filters (which is improper wording to specify that the fuel does not contain additives, the walls of these filter are provided with a catalytic coating) and additive filters.

The authors of the present invention have discovered that the use of an additive filter was especially advantageous when combined with a judiciously chosen fuel injection strategy.

More precisely, the goal of the invention is a method for controlling the emission of pollutants of a combustion engine comprising at least one piston of which the translational displacement delimits a combustion chamber, said engine is associated with an exhaust line comprising, in the flow direction of the exhaust gas, an oxidation catalyst, a NOx reducing catalyst and a particle filter, characterized by the use of a fuel comprising an additive facilitating the combustion of soot during the regenerations of the particle filter, and which according to at least one operating mode of the engine, is injected according to a calibration minimizing the discharge of carbon dioxide by the engine.

In a variant, this operating mode minimizing the discharge of carbon dioxide is obtained by proceeding in all operating zones of the engine—therefore for every torque speed and average effective pressure, including in the so-called “pollution” zone corresponding with low speed and low average effective pressure on the piston—with a main injection of fuel before the upper dead point, in other words more precisely between 20 and 0° crankshaft before the upper dead point, and typically between 12° and 0° in the “pollution” zone. It must be stressed that according to current diesel engine technology, the main injections take place after the upper dead point, for instance between 0° and 10° crankshaft in this pollution zone.

Advantageously, this main injection is preceded by a pilot injection, performed by preference from 2 to 10 degrees crankshaft before the start of the main injection.

In other words, according to the invention, the main injection of fuel is performed at the moment of maximum air compression, in conditions which favor the energy yield of the engine, or conditions which allow for fuel consumption gain and therefore minimizing the quantity of CO₂ produced.

These conditions are normally dismissed by a person skilled in the art because they are accompanied by a significant increase in NOx, which is compensated according to the invention by locating the NOx reduction catalyst upstream of the particle filter.

In a preferred variant of the invention, the operating mode of the engine minimizing the production of CO₂ is employed as soon as the NOx reduction means become active.

In a variant, the injection of fuel is performed in such manner that the fuel/oxidizer mixture has a richness close to 0.7, and more in general, between 0.6 and 0.8.

In a variant, the NOx reduction catalyst and the particle filter are side by side. This avoids perturbation of the gas flow between the two main post treatment means.

In a variant, the oxidation catalyst is impregnated with platinum and palladium, by preference in a molar ratio of platinum/palladium varying between 2 to 1 and 4 to 1. This atypical ratio (a conventional oxidation catalyst normally contains only platinum) stabilizes the molar ratio of NO₂/NOx in the exhaust gas, and consequently improves the performance of the SCR catalyst.

In a variant, an injection of NOx reducing agent takes place between the oxidation catalyst and the NOx reducing catalyst.

Other details and advantageous characteristics of the invention will become clear in the following description with reference to the attached drawings showing:

FIG. 1: a schematic view of an engine and its exhaust gas treatment line;

FIG. 2: a diagram illustrating the principle of shifted injections proposed according to the invention, aimed at minimizing the production of CO₂ and not of NOx as in previous technology.

FIG. 3: a diagram showing in non-dimensioned manner the relationship between the production of CO₂ and the production of NOx, at the exit of the diesel engine;

FIG. 4: a graph illustrating the exhaust gas temperature in different points of the exhaust line, in the hypothesis of driving at average speed lower than 20 km/h.

FIG. 5: a comparative graph showing the cumulated emissions of NOx over a standard driving route, with different engine calibration hypotheses, and with or without post-treatment by a SCR catalyst.

FIG. 6: a graph illustrating the evolution of the average effective pressure (PME) in function of the engine speed and the location of the so-called “pollution” zone.

To be noted that in the rest of the description, unless otherwise stated, the given value ranges include the values at the terminals.

Under NOx nitrogen oxides is understood in particular the oxides of the type protoxide N₂O, trioxide N₂O₃, pentoxide N₂O₅, monoxide NO and dioxide NO₂.

In the framework of the norms applicable to diesel engines commercialized in Europe, the tolerated pollutant emissions are as follows:

Norm CO Nox HC + NOx EURO4 mg/km 500 250 300 EURO5 mg/km 500 180 230 EURO6 mg/km 500  80 170

Therefore, according to the Euro6 norm, applicable in 2014, the tolerated levels of oxide emissions will be reduced by a factor 2.25 relative to the Euro5 levels, applicable in 2009.

In practice, the manufacturers have demonstrated that the norm Euro5, for what concerns nitrogen oxides, can be satisfied for small to medium displacement engines by reducing the emissions at the source through optimization of the combustion chamber geometry and integration of various components of the engine, like for instance a low pressure EGR and very accurate calibration of the engine.

For more powerful engines, or to satisfy even more severe norms, specific post treatment devices are provided such as a SCR catalyst (or “Selective Catalytic Reduction”) which reduces NOx by addition of a reducing agent. The normally used reducing agent is ammonia (NH₃), obtained by thermolysis/hydrolysis of ureum in the exhaust line according to the following reactions:

(NH₂)2CO→HNCO+NH₃:thermolysed at 120° C.

HNCO+H₂O→CO₂+NH₃:hydrolysed at 180° C.

The SCR catalyst serves to promote the reduction of NOx by NH₃ according to the 3 following reactions:

4NH₃+4NO+O₂→4N₂+6H₂O

2NH₃+NO+NO₂→2N₂+3H₂O

8NH₃+6NO₂→7N₂+12H₂O

However, the SCR catalyst is only effective in a temperature range between approximately 180° C. and 500° C., which excludes certain operating conditions of the engines and therefore limits the conversion effectiveness which can be achieved with the SCR principle.

FIG. 1 is a schematic representation of a combustion engine such as a diesel engine according to the invention. The engine comprises at least one piston 1, which travels in translation in a cylinder 2, and the alternative translation movement of the piston is transmitted by connecting rod 3 to crankshaft 4.

Cylinder 2 delimits with piston 1 and cylinder head 5 a combustion chamber in which fresh air is introduced via conduit 6, and admitted into the chamber depending on the position of inlet valve 7. Fuel, for instance, diesel or a bio fuel such as a diester is sprayed in the combustion chamber by injector 8. The mixture is compressed to a sufficiently high pressure for auto-ignition of the air-fuel mixture and the combustion gas is evacuated through an exhaust conduit 9, the opening of which is commanded by an outlet valve 10 and leads into an exhaust collector. From this collector, a conduit 11 serves for the recirculation of a portion of the exhaust gas, a valve called EGR 12 valve controls the exhaust gas flow that is reintroduced into admission.

The here illustrated exhaust line comprises, in the direction of gas circulation, for instance a turbine 13, driven by the exhaust gas and the shaft of this turbine drives for instance a compressor placed in the inlet line of the fresh gas (not shown here). The exhaust gas passes then through an oxidation catalyst 14 which has as primary role to oxidize into carbon dioxide the carbon monoxide contained in the gas exiting the engine.

Downstream of the oxidation catalyst 14, the exhaust gas passes through a NOx treatment catalyst 15. In the case of FIG. 1, this catalyst is an SCR catalyst, containing injection means for a reducing agent such as ureum (injector 16). According to the case, a static mixer 17 can be installed between injector 16 and SCR catalyst 15. A particle trap 18, by preference placed side by side to the catalyst 15, is arranged downstream of this catalyst 11.

As illustrated in FIG. 1, if this SCR catalyst is installed upstream of the particle filter in the exhaust line, these temperatures can be reached relatively easy so that this catalyst is capable of treating very effectively NOx quantities without any relation to the conventional NOx quantities.

Since many years, engine manufacturers have developed fuel injection strategies aimed at minimizing the production of NOx. As illustrated in FIG. 2, in dotted line, these strategies consist in particular in performing an injection in two steps, with a pilot injection preceding the upper dead point and a main injection past this upper dead point.

If, according to the invention, the main injection is advanced to start before the upper dead point (the pilot injection is by preference maintained but shifted likewise), the combustion yield is increased, and therefore the quantity of CO₂ produced. This gain is obtained however at the cost of very significant increase of NOx, in the order of 200 to 250%, a deterioration which seems a total failure according to current practices. The graph of FIG. 3 shows in fact that the gain in CO₂ entails a degradation of NOx and inversely.

FIG. 6 shows also the evolution of the average effective pressure (PME) in function of engine speed for a conventional diesel engine. In addition, in this figure the so-called “pollution” zone is cross hatched, which in this case, corresponds to a speed between 0 and 3000 revolutions per minute, and a PME between 0 and 14. In this cross hatched zone, with high risk of NOx production, a regulation is normally used which starts the main injection after the upper dead point. According to the invention, for all operating points of the engine, the regulation starts the main injection before the upper dead point.

Indeed, this very strong degradation from the point of view of NOx produced is linked to an increase of the temperature in the combustion chamber. This phenomenon is well known, and in traditional manner, counterbalanced by reintroduction at admission of a portion of the exhaust gas in order to dilute the mass of this reactive gas and minimize the temperature in the combustion chamber.

Paradoxically, this temperature increase in the combustion chamber, which is a source of accrued NOx production, has also as consequence an increase of the exhaust gas temperature, so that the SCR catalyst operates in a temperature range that guarantees its optimal function.

Although the SCR catalyst placed upstream of the particle filter absorbs heat, which diminishes the temperature at the entrance of this filter, this temperature remains sufficiently high to allow for the regeneration of the additive filter.

FIG. 4 illustrates FAP regeneration conditions with complementary fuel injection, starting from tests performed by simulating city driving, at average speed lower than 20 km/h, with optimized CO₂ calibration, in other words in very unfavorable conditions for the regeneration of a particle filter but for which a regeneration must nevertheless be possible if the vehicle is used for purely urban driving for instance. As can be seen, the exhaust gas entering the oxidation catalyst (therefore immediately after passing through the turbine if the engine is equipped with a turbo compressor), has a temperature of for instance 250° C. Exiting the catalyst according to the invention (in other words with a molar ratio platinum/palladium of 2/1), the temperature is increased by 200° C. In the SCR catalyst, the thermal losses ensure that the temperature is not higher than 350° C., in other words a temperature lower than the priming temperature necessary for a regeneration in the case of a non additive filter, but well higher than the temperature in case of an additive filter.

As previously indicated, the recommended regulation of the engine according to the invention, with a main fuel injection centered around the dead point, leads to a significant increase of NOx production at the exit of the engine. The graph of FIG. 5 shows that nevertheless, the NOx emission rate of the vehicle can be perfectly controlled.

In this FIG. 5, a standardized driving route is represented of 20 minutes (1200 sec), by imposing speeds 1 (ordinate axis to the left of the figure). This distance corresponds to the European homologation cycle MVEG.

In the same graph, referring to the ordinate axis to the right of the figure, are also indicated the NOx emissions in grams, cumulated over the whole route.

In 2, the emissions are shown obtained in the hypothesis of an engine with NOx optimized calibration, according to prior art, in order to satisfy the norm Euro5. At the end of the route, approximately 2.5 g of NOx has been emitted.

If the same engine is now equipped with a SCR catalyst, the emissions are reduced under 1 g (curve 3), either on this side of the limit foreseen for the norm Euro6, indicated by a discontinuous line, and slightly lower than 1 g. We observe a very high NOx reduction efficiency at the end of the cycle (>90%) due to the high exhaust temperatures.

Installing the same engine again in a vehicle without NOx post treatment, and adopting a calibration according to the invention, with optimized CO₂ which can be activated at any time (in the here illustrated case at 800 sec), as shown by curve 4, the cumulated NOx emissions move away very quickly from this standard, and reach at the end of the route a level higher than 3.5 g (although this optimized calibration is employed only in the last third of the route, once all of the depollution equipment is properly operational).

If now according to the invention and as illustrated in curve 5, NOx post treatment equipment is available upstream of the particle filter, the standard calibration (optimal NOx) is again used for the start of the cycle (from 0 to 800 sec) because the NOx conversion efficiency is lower, then the new calibration—optimal CO₂—is used at the end of the cycle (from 800 to 1200 sec) because the Nox conversion efficiency is better, we observe in curve 6, that using a fuel containing an additive promoting the combustion of soot during the regenerations of the particle filter, the reduction efficiency—according to at least one operating mode of the engine associated with a calibration minimizing the discharge of carbon dioxide by the engine—can remain so that notwithstanding everything, the regulatory emission level is satisfied. In these conditions, a significant CO₂ benefit is obtained. The average gain over the whole cycle is situated around 4 to 5%, or according to the vehicle considered a gain of 5 to 8 g of CO₂ per km.

In the preceding example, the optimized CO₂ calibration is activated after 800 sec. In more general manner, the optimized calibration according to the invention will be activated according to a predefined threshold corresponding to a given efficiency state of the post-treatment means, this state can be estimated in known manner starting from models based for instance on engine tests, models which advantageously, could take into account the aging of these means. 

1. A method for controlling the emissions of pollutants by a combustion engine comprising at least one piston of which the displacement in translation defines a combustion chamber, said engine associated with an exhaust line comprising, in the direction of the exhaust gas flow, an oxidation catalyst, a NOx reduction catalyst and a particle filter, wherein the method comprises: providing a fuel containing an additive promoting the combustion of soot during the regenerations of the particle filter; operating the engine in a selected operating mode; and injecting the fuel containing the additive into a combustion chamber of the engine according to a selected CO₂ calibration, thereby minimizing the discharge of carbon dioxide by the engine.
 2. The method according to claim 1, wherein said operating mode minimizing the discharge of carbon dioxide is obtained by injecting a main injection of fuel between 20° and 0° crankshaft before the upper dead point, at any torque speed and effective average pressure of the engine.
 3. The method according to claim 2, wherein the main injection is preceded by a pilot injection.
 4. The method according to claim 3, wherein said pilot injection takes place between 2 and 10° crankshaft prior to the main injection.
 5. The method according to claim 1 wherein the injection of fuel is performed with a mixture with richness between 0.6 and 0.8.
 6. The method according to claim 1 wherein the operating mode of the engine minimizing the discharge of carbon dioxide is activated as soon as a NOx reduction means is active.
 7. The method according to claim 1 wherein the catalytic reduction of NOx is obtained by means of a reducing agent injected in the exhaust line upstream of the NOx reduction catalyst.
 8. The method according to claim 1 wherein the NOx reduction catalyst and the particle filter are located side by side.
 9. The method according claim 1 wherein the oxidization catalyst is impregnated with one of platinum and palladium.
 10. The method according to claim 9, wherein a molar relationship of the one of platinum and palladium is 2 to
 1. 